In a typical nuclear power plant, heat energy is generated by fissioning uranium fuel within a reactor core. The heat energy is then collected in water, also known as coolant, and is carried away from the reactor core either as steam in boiling water reactors or as subcooled water in pressurized-water reactors. In a pressurized-water reactor, the sub-cooled water in the primary cooling loop is used to transfer heat energy to a secondary loop for the creation of steam. In either a boiling-water or pressurized-water installation, steam under high pressure is then used to transfer the nuclear reactor's heat energy to a turbine that mechanically turns an electric generator.
The reactor core for a nuclear power plant typically includes many fuel assemblies. A fuel assembly contains a group of sealed fuel pins, or rods, each filled with uranium oxide pellets. A thin-walled metal tube, known as cladding, forms the outer jacket of each fuel pin. The cladding prevents the release of fission products into the coolant. Stainless steel, and zirconium alloys are common cladding materials. The fuel pins are held in place by end plates and supported by metal spacer-grids to brace the pins and maintain the proper distance between them. In operation, the coolant flows in sub-channels between each of the fuel-pins within each of the fuel assemblies to carry the heat energy, extracted through the nuclear chain reaction process, away from the core.
Marketplace pressure has driven the need for reducing power plant operating costs and increasing capacity factor, i.e., the ratio of the actual electrical energy to the energy that could have been generated at continuous full-power operation during the same period. In an attempt to meet the marketplace demands, nuclear power plants are shifting toward more efficient core designs. One such evolution of reactor core design is the transition from traditional checkerboard loading patterns of fuel assemblies to Ring-of-Fire, or Saturn, loading patterns. The Ring-of-Fire designs typically exhibit longer cycle length, higher average enrichments, and higher fuel duty than checkerboard designs.
The higher fuel duty of the Ring-of-Fire loading patterns advantageously results in increased nucleate, or sub-cooled boiling, enhancing the efficiency of heat transfer. The onset of sub-cooled boiling indicates the location where the vapor can first exist in a stable state on the surface of the fuel pins without condensing or vapor collapse. As more energy is input into the liquid (i.e., downstream axially) these vapor bubbles can grow and eventually detach from the fuel pin surface and enter the coolant.
Although the efficiency of heat transfer is enhanced, unfortunately the higher fuel duty also causes deleterious crud and oxide deposition on cladding surfaces of the fuel pins. The term “crud” is an acronym for Chalk River Unidentified Deposits first discovered as black, highly radioactive material covering fuel assemblies at the Chalk River nuclear reactor. CRUD has now become a standard industry term referring to minute, solid, corrosion products that travel into the reactor core, become highly radioactive, adhere to the fuel pins of the fuel assemblies, and also flow out of the reactor into other systems in the plant. Excessive crud deposition inhibits heat transfer, thus increasing clad temperature and oxide layer growth rate. Excessive crud deposition also concentrates lithium, accelerates corrosion leading to fuel failures, and is a critical contributing factor to the onset of axial offset anomaly (AOA).
AOA is a major impediment to increases in reactor fuel performance. AOA is an unexpected deviation in the reactor core axial power distribution during operation from the predicted distribution. Since AOA bears an immediate threat to a nuclear power plant's competitiveness, it is highly desirable to develop strategies to solve or mitigate the AOA problem. Currently, nuclear power plant operators are avoiding AOA though reload management, which is an inefficient and costly remedy to the problem.
Due to the deleterious effects of excessive crud deposition in a reactor core, and its contribution to the onset of axial offset anomaly (AOA), what is needed is an approach for minimizing the adverse impact of crud deposition in a reactor core.